There are situations in which heat must be transferred from a locale of heat generation to a locale of heat rejection under circumstances in which insufficient energy exists to operate a conventional heat transfer system. This occurs in spacecraft environments where large amounts of heat must be rejected to ensure the proper operation of the spacecraft and its systems. Locales of heat generation in a spacecraft include the on-board electronics and exterior surfaces facing the sun, while locales of heat rejection include exterior surfaces not facing the sun and areas requiring heat, such as a crew's cabin.
One system which transfers heat efficiently with little or no external power requirements is the capillary pump loop (CPL) heat pipe system. A CPL heat pipe system is a two-phase heat transfer system which utilizes a vaporizable liquid. Ammonia and the FREONs have been found to be suitable working liquids. Heat is absorbed by the liquid when its phase changes from a liquid state to a vapor state upon evaporation, and heat is released when condensation of the vapor occurs. The CPL heat pipe system includes a heat pipe containing a capillary structure, such as a porous wick, and a continuous loop. The continuous loop provides a vapor phase flow zone, a condenser zone, and a liquid return zone.
The key factor affecting the efficiency of the heat transfer by a CPL heat pipe system is the selection of the working fluid. In turn, the wick employed in the loop must be compatible with the working fluid. Besides being compatible with the working fluid, good wicks must have uniform porosity, small pore size and high molecular weight. Compatibility must be both chemical and physical. The wick must not swell, shrink or shed particles. Uniform porosity is required to achieve uniform flow and a uniform pressure head at the outside surface of the wick. The pore size of the wick should be very small, because as the pore size decreases, the capillary pressure, i.e., fluid static height or pumping action which the wick can generate, increases, and the amount of heat which can be transferred also increases. However, as the pore size decreases, the permeability of the wick to radial and longitudinal fluid flow also decreases. Also, the tendency for the wick to clog may increase. Thus, for maximum heat transfer efficiency, a wick material offering both small pore size and high permeability is preferable. Other factors are also to be considered in selecting a wick material. The wick material should be resistant to chemical attack by the working fluid, and it should not contaminate the fluid chemically or physically generate particulates. Chemical contamination of the fluid will change its evaporation characteristics, and it may produce gas bubbles which will accumulate and enlarge in the condenser zone and eventually block it. Particulate contamination will also cause blockage of the continuous loop. Furthermore, it is desirable for the wick material to be resistant to degradation by heat, and to be cold resistant for use in low temperatures heat transfer applications. Generally, it is desirable for the wick to operate from -70.degree. C. to +70.degree. C. Lastly, the wick material should be easy to machine so that it can be made to conform to a heat pipe having any geometrical shape, and flexible so as to be vibration resistant.
Heat pipe wicks have been heretofore fabricated of various types of materials in an attempt to achieve ammonia and FREON compatible wicks. One type of material is a brillo-like metal wire mesh, but no capillary action was achieved. Examples of metals used are copper, stainless steel, and aluminum. Wire mesh wicks are made by knitting, felting round wire, and by stacking corrugated flat ribbon wire. They generally have pores of nonuniform size, which results in the poor and uneven generation of capillary pressure along the length of the wick, and they are subject to chemical attack by corrosive fluids. They are also very friable, which results in the fluid being contaminated with particulates, and they can chemically contaminate the fluid.
Another type of wick material is a sintered metal wick. Examples of metals used in sintered metal wicks are copper, oxidized stainless steel, molybdenum, tungsten, and nickel. These wicks are generally constructed in tubular or flat sheet form by heating metal powder or metal slurries on a removable, cylindrical or flat mold mandrel. Wicks produced by this method are usually friable, and have pores of uneven size. They are also subject to chemical attack by corrosive fluids, and they can chemically react with chemically active fluids to contaminate them.
Heat pipe wicks may also be constructed of sintered ceramics. Sintered ceramic wicks, however, are extremely friable, and they exhibit poor capillary performance. Additionally, they are physically and chemically degraded in use, and they are difficult to produce in tubular form.
Two other types of wick materials are cloth wicks and glass fiber wicks. Cloth wicks are generally formed by stacking disks of cloth cut out of a sheet to form a cylinder. Cloth wicks are subject to attack by corrosive fluids, and they produce particulates and fibers in use. Glass fibers, on the other hand, are not subject to attack by corrosive fluids. However, they are very brittle, hard to form into a desired shape, and they cannot be greatly stressed or strained in use without breaking.
One particular material which has been used as a heat pipe wick is a felted ceramic comprised of 50% SiO.sub.2 and 50% Al.sub.2 O.sub.3. Rings of this material are cut out of a sheet and stacked together to form a cylinder. This material is extremely friable, and it exhibits poor capillary performance. It also produces particulates during use and is subject to chemical attack by corrosive fluids.
Of all the known CPL wicks, including those noted above, none have been found to be suitable for use with anhydrous ammonia and the FREONS, such as FREON 11, which are the most effective refrigerants known.